Implement IOSHttpRequest and IOSHttpResponse http interfaces and use instead of OSHttpRequest/OSHttpResponse.

This is required for the substitution of different HTTP servers or the newer HttpServer.dll without having to commit to a particular implementation.
This is also required to write regression tests that involve the HTTP layer.
If you need to recompile, all you need to do is replace OSHttpRequest/OSHttpResponse references with IOSHttpRequest/IOSHttpResponse.

OpenSim/Region/DataSnapshot/DataRequestHandler.cs

@@ -74,7 +74,7 @@ namespace OpenSim.Region.DataSnapshot
         }
 
         public string OnDiscoveryAttempt(string request, string path, string param,
-                                         OSHttpRequest httpRequest, OSHttpResponse httpResponse)
+                                         IOSHttpRequest httpRequest, IOSHttpResponse httpResponse)
         {
             //Very static for now, flexible enough to add new formats
             LLSDDiscoveryResponse llsd_response = new LLSDDiscoveryResponse();